The present invention relates to a MEMS microphone with a built-in textile material protective screen.
As it is known, the most common types of microphones for acoustical devices are the so-called “ECM” (Electret and Capacitor Microphone) and “MEMS” (Micro Electrical Mechanical System) microphones.
The second microphone type is a small device for installation in cellular phones and other hand-held acoustical devices, and has a typical size of just 7×4×3 mm.
A MEMS microphone comprises a microphone metal body which is made by the so-called “packaging” assembling process, in which different material layers, also comprising metal materials, are overlapped onto one another in a fully automatic manner.
After having soldered the microphone inner electric circuit components, the microphone thus made is separated from a disposable microphone plate, by cutting through the perimeter of the latter.
By a single cutting operation, a plurality of microphones are cut away.
Before cutting, all the microphones are electrically tested and the rejected ones are automatically separated from the good ones.
The microphones thus made have basic features, the most important of which is their very small size.
The microphone metal body is a good heat conductor.
The above process generates a high heat amount as the microphone inner portion is soldered: accordingly it is necessary to perform a quantitative and qualitative control on 100% of the made microphones at the outlet of the making lines.
As it is also known, the above microphone comprises a plurality of metal pins, typically six, to be soldered to the PCB (i.e. the printed circuit board, or the so-called motherboard), as the final audio device is assembled.
Typical methods for mounting MEMS microphones on a PCB will be disclosed hereinafter.
A MEMS microphone may have a MEMS pattern of a type either mounted on the top or on the bottom, with reference to the mounting position of the microphone on the PCB (either the inner or outer face thereof).
Both the mounting patterns may be used for mass producing acoustical devices, the selection depending on the target final acoustic device architecture.
The microphone opening must be arranged directed toward the sound source, to pick up sound waves for a satisfactory microphone operation.
Depending on the position of the microphone on the PCB or motherboard, the microphone opening may vary.
In fact, for top mounted MEMS, the opening is held on the face of the PCB opposite to the coupling pins to be soldered to the PCB.
On the contrary, for MEMS microphones mounted on the bottom face, the PCB is drilled and the coupling pins are soldered to contacts all arranged about the hole.
Thus, the MEMS opening should be arranged on the same face as that of the coupling pins, i.e. facing the outside sound source.
Accordingly, it should be apparent that the above different positions affect the heat amount flowing to the opening during the pin soldering operations: more specifically, for bottom mounted MEMS, the opening will be arranged nearest to the connecting pins and the heat flow will be a maximum one.
This represents a problem when a protective screen is applied directly at the opening position, as it should be apparent hereinafter.
With respect to the microphone protection, a basic need is that of preventing small particles and water drops from entering the microphone, thereby causing failures and a decay of the acoustical response.
Such a protection may be achieved by a protective screen arranged near the MEMS opening.
Fabric material and porous media screens are generally used as a protective barrier against a penetration of particles.
In the acoustical field, an open mesh fabric material (net) is frequently used.
The mainly used material consists of a fabric comprising synthetic single thread yarns, with a net mesh opening from 20 to 100 microns.
A polyester polymer (PET) is conventionally used.
An example of such a fabric material is the well known “Acoustex B160” fabric, having a 21 micron opening mesh pattern with a density of 190 threads/cm, which is spun from a 31 micron single-thread PET yarn, having an acoustically impedance of 160 MKS Rayls, and made, for example, by the company Saati SpA.
When a protective screen is applied, a typical solution is that of using a die-cut independent portion, made of a woven net with a foam and adhesive material, connected near the opening through the acoustical device outer surface.
Such provision of a separated component prevents the screen from being exposed to the preliminarily soldering operation heat.
In case of a top mounted MEMS, a prior solution is that of applying a die-cut portion to the inner face of the device outer body.
Such a component is constituted of the fabric material protective screen with an addition of a double adhesive ring element and optionally a thin foamed layer operating as a hanger layer.
Since the die-cut portion remains attached to the cellular phone outer body, or other audio device, said die-cut portion provides a good protection.
However, this approach is affected by the serious drawback that it adds an independent component which must be necessarily installed during the device final assembling, thereby increasing the device making time and cost.
Moreover, the MEMS microphone is left unprotected during the intermediate assembling steps, from the printed board soldering step to the plastics material body closing step.
Considering the comparatively high number of defective pieces of conventional MEMS making methods, the lack of such a protection in the mentioned assembling steps is very dangerous.
The bottom mounted MEMS have less exposed openings, which, however, must be protected against fine particle penetrations.
At present, on the printed board opposite face, a die-cut portion consisting of a synthetic protective screen plus an adhesive is applied, which efficiently protects the MEMS opening against a very small particle intrusion.
Such a solution is satisfactory, even if not perfect, since a laterally open space is left; however, it has the same drawback as that of the top mounted MEMS systems, that is an additional die-cut portion assembling step and a lack of microphone intermediate protection during the device assembling operation.
In addition to the above, in the so-called “packaging” and assembling steps, the MEMS microphones are heated at least two times.
MEMS may be also made by other making methods, some of which use the “reflow” process generating a large amount of heat in the microphone body, thereby the microphone body may achieve a critical temperature.
Moreover, temperature data may change depending on the MEMS making method, mainly as said MEMS are soldered to the PCB at either upward or downward directed connection soldering points during the final device assembling operating step.
In this connection it should be moreover pointed out that a MEMS has a very small size and is mainly made of a metal material, which is a further critical condition from a thermal standpoint.
The temperature generated in making a MEMS may have a value of 300° C., which is dangerous for the protecting screen material.
At present, the above protecting screen or barrier is usually made of a polyester (PET, polyethyleneterephtalate), that is a polymer having a melting temperature from 240 to 260° C., thereby a direct exposition to a higher temperature could damage or destroy the synthetic mesh screen.
Thus, for the above reason, the above mentioned die-cut portion is applied near the MEMS, but not therewithin.
The screen application per se is carried out in a second operating step, at the end of all the heating processes, that is after having made the MEMS and soldered it.
This is the main limitation of the prior MEMS making methods, that is a very complex assembling, a lack of protection during the intermediate processing steps and an unsatisfactory and non perfect protection during the use.
As above disclosed, prior approaches are not ideal both for top mounted and bottom mounted MEMS, because of the requirements of assembling an additional component greatly increasing the making cost.
Moreover, the logistic properties are also deteriorated because of the provision of the mentioned additional component to be added and separately handled.
Moreover, it should be further pointed out that the protective screen is applied only when the final acoustical device has been fully assembled, or at least after having soldered the microphone on the PCB, and usually at a different making place of another customer.
Thus, during the making, storing, transporting (if provided) and final PCB circuit soldering, the MEMS are left in an unprotected condition.
A theoretical ideal solution would be that of providing protection to the MEMS per se.
In other words, the protective screen should be made as a single body with the MEMS, thereby in the following assembling steps it would be possible to “forget” the screen presence.
In actual practice, such an idea is impracticable because of temperature limitations.
It would be very advantageous to fully install the protective screen before soldering the microphone in its target position.
This means that the synthetic screen should endure at least the heat generated in the soldering operation, which is a rather critical thing, mainly for bottom mounted MEMS.
In fact, in a bottom mounted MEMS, the microphone opening is facing the PCB very near to the coupling contacts to be soldered to the PCB.
Thus, by considering the minimum distance and high conductibility of these metal portions, the screen melting temperature is undesirably achieved during the soldering operation.
In other words, the bottom mounted pattern is the most sensible to heat at the microphone opening region.
If it is desired to mount the protecting screen near the opening, then the screen must endure the soldering heat.
For a top mounted MEMS, the distance between the coupling pins and the opening is slightly larger and, accordingly, the thermal stress of the soldering operation is smaller than that of the bottom mounted MEMS.
In some favorable conditions, a prior polyester screen is satisfactory from a thermal standpoint.
However, for assuring a proper protective barrier in all the intended applications, it would be desirable to have a material with a larger acceptable temperature threshold.